Date of Award
Doctor of Philosophy
Genetics, Development and Cell Biology
Philip W. Becraft
Brassinosteroids (BR) are a class of phytohormones with many important roles in plant growth and development. Compared to other model organisms BR signaling is poorly understood in maize. To study BR signaling's role in maize growth and development we suppressed two members of the BR signaling pathway, BRI1 and BIN2, by RNAi approach. BLAST searches and phylogenetic analyses suggested that there are two BRI1 copies and three BRI1-like genes (BRL) homologs in maize. bri1b, which is located on chr 5, was incomplete in databases, but via subsequent cloning and sequencing was found to be an intact and expressed gene. These two closely related bri1 homologs, named bri1a and bri1b, share 93% amino acid identity and 95% similarity. Suppression of BRI1 homologs lead to dwarf plants with upright, dark green, and thicker leaves. Blade/sheath boundary was disrupted in bri1-RNAi lines as well. Increased and localized accumulation of BES1: YFP, a BR responsive marker, in the developing ligule/auricle region suggests that BR signaling is central to auricle development. For BIN2, a GSK3-like kinase, phylogenetic analyses suggested that there are 10 homologs in maize. NJ method divided BIN2 homologs into four clades with 5 members belonging to "Clade II", whose members perform primary BR signaling functions in Arabidopsis. All these members had SIWID domain, which is unique to Clade II. bin2-RNAi plants had shorter stature, which was unexpected because BIN2 is a negative regulator of BR signaling. However tassel internodes and leaves were longer as expected for increased BR signaling. In contrast to bri1-RNAi plants, bin2-RNAi plants had larger auricles. bin2-RNAi plants also had changed kernel traits, too. When bin2-RNAi lines used as female, kernels on the cob independent of their genotype were larger, opaque, and had pointy ends. SEM analysis revealed that these opaque kernels were lacking a normal horny endosperm and their starch granules were larger, less compact, and non-spherical compared to non-opaque kernels granules. The bin2-RNAi transgene rescued the mild bri1-RNAi phenotype consistent with the predicted position of BIN2 downstream of BRI1 in the signaling pathway. A kinematic analysis on leaf #4 of both bri1-RNAi and bin2-RNAi showed that leaf elongation rate (LER) was increased 14% in bin2-RNAi lines, whereas decreased 17% in bri1-RNAi lines. Both cell division and cell elongation contributed to the effects on leaf length in each line. These results suggest that BR signaling has similar as well as unique roles in maize development compared to other species.
Kir, Gokhan, "Regulation of shoot development in maize via brassinosteroid signaling" (2015). Graduate Theses and Dissertations. 14379.